The present invention relates generally to an engine control system for controlling an internal combustion engine, which system includes means for detecting an occurrence of misfire in a cylinder of the engine on the basis of the pressure therein. More particularly, the invention is concerned with a method and an apparatus for detecting failure or malfunction of a pressure sensor which is installed in association with an engine cylinder for the purpose of detecting the pressure therein.
In general, internal combustion engines (hereinafter also referred to as an engine for short) typified by four-cylinder four-cycle engines such as gasoline engines for motor vehicles and the like comprise a plurality of cylinders (e.g., four cylinders) and operates in four cycles including suction (intake), compression, power and exhaust strokes, respectively. In recent years, a microcomputer-based engine controller is increasingly adopted in this kind of engine with the aim for realizing optimal control of ignition timings of igniters provided for engine cylinders, a fuel injection sequence and other factors participating in the engine operation.
For effectuating the optimal engine control, the microcomputer-based engine controller fetches therein not only the signals representing various operating conditions and running states of the engine but also a reference position signal for each cylinder in synchronism with the engine rotation and cylinder identification signals identifying the individual cylinders for the purpose of controlling on the per-cylinder basis the cylinder operation at the optimal timing by detecting operating positions (crank positions or angles) thereof. As the means for generating the reference position signal and the cylinder identification signal, there is conventionally employed an angular signal generator designed for generating a synchronous signal by detecting an angular position of a camshaft or a crankshaft of the engine.
Through the ignition control for each cylinder, a fuel/air mixture compressed by a piston must undergo explosive combustion, being fired by a spark produced by an spark plug at a proper timing (or time point). In this conjunction, it is known that the combustion sometimes fails to take place at the optimal timing, depending on the engine running states or other factors, which necessarily results in an insufficient output torque. Furthermore, nevertheless of the ignition control, it may sometimes occur that no combustion takes place within a cylinder, depending on the type of the fuel, state of the spark plugs and other factors, as a result of which abnormal load is imposed on the other cylinder(s), which may eventually lead to serious problems such as injury or damage of the engine as well as discharge of uncombusted gases to the atmosphere.
Such being the circumstances, in order to assure safe operation of the engine, it is required to detect on an ignition-cycle basis whether or not combustion actually occurs at the optimal timing in each cylinder. To this end, there has been proposed a system for detecting the cylinder pressure (i.e., pressure within the cylinder) during the explosion or power stroke following the ignition, to thereby discriminatively identify the occurrence of combustion or misfire (i.e. non-occurrence of the combustion). By way of example, it is known to perform feedback control of the ignition timing by making use of a deviation from a peak position corresponding to a peak of the cylinder pressure so that the peak position or the corresponding crank angle takes place at a crank angle of 15.degree. after top dead center (TDC). Additionally, a misfire detecting system is also employed which is designed to determine the occurrence of misfire unless the cylinder pressure rises to a sufficiently high level during the power stroke, thereby indicating the occurrence of misfire in the associated cylinder in order to allow the corresponding engine control to be performed.
For a better understanding of the invention, a typical example of the misfire detecting systems will be described by reference to FIG. 13 which shows schematically the structure of an internal combustion engine equipped with an engine controller.
In this figure, a reference numeral 1 denotes generally a cylinder constituting a major part of the engine. The cylinder 1 includes a combustion chamber 2, a spark plug 3 mounted within the combustion chamber 2, a piston 4 adapted to be driven under explosive combustion of a fuel/air mixture within the combustion chamber 2, an intake port 5 for supplying the fuel/air mixture to the combustion chamber 2, an exhaust port 6 for discharging an exhaust gas resulting from the combustion, an intake valve 7 in the intake port 5 for controlling the fuel supply to the combustion chamber 2, and an exhaust valve 8 in the exhaust port 6 for controlling the discharge of exhaust gas from the combustion chamber 2.
The spark plug 3 is composed of a center electrode electrically connected to an ignition coil (described hereinafter) and a grounded electrode disposed in opposition to the center electrode. Needless to say, a four-cylinder engine, for example, includes four cylinders each of the structure described above.
Turning back to FIG. 13, a referenece numeral 9 denotes a fuel injector installed within the intake port 5 for supplying to the cylinder a fuel/air mixture of an air-fuel ratio which is determined by an amount of air flow controlled by a throttle valve (not shown) whose opening degree in turn is controlled by an accelerator pedal (also not shown). Further, an orifice 2a is formed in a wall portion of the cylinder defining the combustion chamber 2. A pressure sensor 10 detects the cylinder pressure (i.e. pressure within the cylinder) by way of the orifice 2a. An ignition coil 11 includes a primary winding and a secondary winding having an output terminal connected to the center electrode of the spark plug 3. A power supply source 12 applies a voltage of a minus (negative) polarity to an input terminal of the ignition coil 11. An ignition device 13 is connected to an output terminal of the primary winding of the ignition coil 11.
Finally, a microcomputer-based engine control unit 14 (also referred to as the engine controller or ECU for short) controls the operations of the engine as a whole including those of the intake valve 7, the exhaust valve 8, the fuel injector 9 and the ignition device 13. The engine controller unit or ECU 14 incorporates as constituent parts thereof a threshold generating circuit for generating a threshold level signal serving as a reference upon making a decision as to the occurrence of misfire as well as various arithmetic/processing units. The ECU 14 fetches therein a voltage signal respresentative of the cylinder pressure P outputted from the pressure sensor 10 together with a referenece position signal representative of reference cylinder positions and other various signals representing the engine running states.
For allowing the cylinder pressure signal P to be fetched by the ECU 14 at a predetermined time point(s) during the power stroke, there is provided an angular position sensor (not shown) for generating a reference position signal corresponding to a reference crank angle(s). The angular position sensor may include a rotatable slitted member having a slit formed at a position corresponding to the predetermined time point or timing in the explosion stroke, wherein the slit position corresponding to the predetermined timing may be set at a crank angle at which a remarkable difference in the cylinder pressure develops in dependence on occurrence or non-occurrence of explosive combustion. To this end, the crank angle may be set to an angle selected from a range of 10.degree. to 90.degree. after top dead center. In this connection, the crank angle before reaching the top dead center will symbolically be represented by affixing a prefix "A" to the angle value while the crank angle after passing the top dead center will be represented by affixing a prefix "B".
FIG. 14 is a view for graphically illustrating in what manner the cylinder pressure P changes as a function of the crank angle .theta.. In this figure, a symbol TDC represents top dead center at which the crank angle .theta. assumes a value of zero, .theta..sub.Pmax represents a peak crank angle corresponding to a maximum value Pmax of the cylinder pressure P, and .theta..sub.R represents an optimum peak crank angle.
Now, referring to FIG. 14 along with a flow chart shown in FIG. 15, description will be made of the control operation performed by the ECU 14 for the internal combustion engine shown in FIG. 13.
During two reciprocations of the piston 4, there take place within the combustion chamber 2 four cycles of a suction stroke, a compression stroke, an explosion or power stroke and an exhaust stroke, respectively. In the course of the four-cycle operation, the ECU 14 optimally controls the amount of fuel supplied by the fuel injector 9 on each intake stroke, the ignition timing for the spark plug 3 and the like in accordance with the desired engine running state to be realized.
More specifically, when the fuel/air mixture is supplied to the combustion chamber 2 from the intake port 5 by opening the intake valve 7, the ECU 14 optimally controls the amount of fuel injected through the fuel injector 9 as well as the amount of air supplied through the intake port 5 in accordance with the opening degree of the throttle valve actuated by the accelerator pedal.
After the fuel/air mixture is compressed by the piston 4 within the combustion chamber 2, the ECU 14 drives the ignition device 13 at a predetermined timing to thereby electrically energize the primary winding of the ignition coil 11, as a result of which a high voltage of negative polarity is applied to the center electrode of the spark plug 3 from the secondary winding of that coil 11. Thus, electric discharge in the form of a spark takes places between the center electrode and the grounded electrode, firing the compressed fuel/air mixture in the combustion chamber 2 for explosive combustion. Usually, the ignition timing is so controlled as to occur at a crank angle close to top dead center (TDC), i.e. the crank angle of approximately zero.
Upon occurrence of the combustion or explosion, the cylinder pressure P within the combustion chamber 2 becomes high. Needless to say, the cylinder pressure P is constantly detected by the pressure sensor 10. However, if no explosion or misfiring takes place, the cylinder pressure remains at a relatively low level. Of course, the cylinder pressure P assumes the maximum value Pmax at the peak crank angle .theta..sub.Pmax. However, in order to make the maximum output torque available, it is desirable that the peak crank angle .theta..sub.Pmax coincide with the optimal position .theta..sub.R (e.g., 15.degree. after TDC).
On the basis of the cylinder pressure detected by the pressure sensor 10, the ignition timing feedback control is so performed as illustrated in FIG. 15. More specifically, in step S101, the cylinder pressure P is detected by the pressure sensor 10, whereon the peak crank angle .theta..sub.Pmax, at which the cylinder pressure P assumes the maximum value Pmax, is determined by the ECU 14 on the basis of the sensor output waveform representative of a change in the cylinder pressure P (such as shown in FIG. 14).
Subsequently, in step S102, a difference or deviation .DELTA..theta..sub.P of the peak crank angle .theta..sub.Pmax from the optimal position .theta..sub.R is determined as follows: EQU .DELTA..theta..sub.P =.theta..sub.R -.theta..sub.Pmax
Next, in step S102, the deviation .theta..sub.P is multiplied by a feedback gain correcting coefficient K (.ltoreq.1) to arithmetically determine an ignition timing correction quantity .DELTA..theta..sub.ig in accordance with the following equation: EQU .DELTA..theta..sub.ig =K.times..DELTA..theta..sub.P
Finally, in step 103, the feedback control quantity for controlling the ignition timing is arithmetically determined by the ECU 14 on the basis of the correcting quantity .DELTA..theta..sub.ig as follows: EQU .theta..sub.ig =.theta..sub.MAP +.DELTA..theta..sub.ig
where .theta..sub.MAP represents a value of the ignition timing previously established in dependence on the operating state of the engine or other factors while looking at a map or table.
At this juncture, it should be mentioned that the pressure sensor 10 is composed of a metal diaphragm or membrane disposed on the side exposed to the pressure within the cylinder and a piezoelectric circuit for outputting the sensed cylinder pressure P in the form of an electrical signal. Consequently, when a failure such as short-circuit, disconnection, wire breakage or the like fault (referred to as minor failure) occurs in the circuit of the pressure sensor 10, the cylinder pressure P as detected exhibits an abnormal value, which will ultimately results in that the feedback control illustrated in FIG. 15 is prevented from being properly and correctly carried out. On the other hand, if injury of the metal diaphragm or membrane (referred to as heavy failure to distinguish it from the minor failure mentioned above) should take place, not only the feedback control is rendered impossible but also such unwanted situation may be incurred that combustible gas within the combustion chamber 2 diffuses into the interior of the pressure sensor 10 through the pressure-responsive diaphragm to thereby aggravate the injury of the pressure sensor. Besides, there may arise an undesirable event such as leakage of the combustible gas to the ambient through the pressure sensor 10 injured.
In this conjunction, it will be noted from the foregoing that the conventional internal combustion engine control apparatus is incapable of detecting such failures of the pressure sensor 10 as mentioned above. In other words, it is impossible for the ECU to accurately detect the cylinder pressure in case a failure has occurred in the pressure sensor, thus involving erroneous control on the ignition timing due to incorrect detection of the cylinder pressure.